The invention relates to a hydraulic control device having a pressure connection and a tank connection and having a metering pump unit that comprises at least two metering pumps connected hydraulically in parallel and operable mechanically in parallel and also a cut-off valve in a hydraulic connection between the two metering pumps, the cut-off valve being flange-mounted on a metering pump and having in a housing a slider member which, under pressure from the pressure connection, is movable axially towards the metering pump.
Such a control device is preferably used to steer vehicles. The metering pump unit is in this case connected by way of a directional valve to a steering motor. Both the metering pumps and the directional valve are connected to a steering handwheel or to a comparable device. When the steering handwheel is turned, the directional valve is displaced in the desired direction and the metering pump unit continues to convey hydraulic fluid until the steering motor has reached the desired position. In normal undisturbed operation, both metering pumps are operative. They can convey a correspondingly large volume of hydraulic fluid, which allows the steering motor to respond rapidly to movements of the steering handwheel.
When the pressure at the pump connection drops or fails for any reason, for example, because the supply pump responsible is defective, the metering pumps can be used in a so-called "emergency operation" also as auxiliary pumps. In that case the metering pumps are then used to generate the pressure of the hydraulic fluid. For this, the necessary energy has to be introduced by way of the steering handwheel, that is, generally by human muscle power. To relieve the operator of the effort involved in this, it is known from DE 22 28 531 C2 to provide a cut-off valve, by means of which, when the pressure at the pump connection fails, the second metering pump is switched off. The operator then has to operate only one metering pump. For the same movement of the steering motor he then has to turn the steering handwheel further, but the effort involved is less.
It is precisely with vehicle steering systems that it is desirable always to keep the necessary installation space as small as possible, and also to make the parts required for the steering as light-weight as possible. For that reason, in the control device according to DE 22 28 531 C2, the two metering pumps are flanged to one another. In addition, the cut-off valve is also flange-mounted on one of the metering pumps. In normal operation, when the pressure connection is supplied with pump pressure of the supply pump, the slider member of the cut-off valve is pressed by this pressure from the pressure connection against the adjacent metering pump. Since it is desirable to use no more material than necessary for the housing of the metering pump, this may possibly result in deformation of the housing of that metering pump under the pressure of the slider member. Although this does not impair the ability of the metering pump to function, the wear and tear that occurs is in some cases considerable, and can shorten the service life of the control device. This wear and tear occurs not only through the axial wear in the gear assembly adjacent to the cut-off valve. Because of the increased friction, both pumps have to be acted upon with an increased pressure and, because of the relatively high pressure difference over the gear wheels, this leads to greater wear in the two gear assemblies.